Integrative taxonomic description of Stylocephalus epistupaformis n. sp. (Apicomplexa: Eugregarinorida) parasitising the darkling beetle Gonocephalum depressum Fabricius, 1801, in West Bengal, India.

The genus Stylocephalus Ellis, 1912 (Apicomplexa: Eugregarinorida) exhibits wide distribution and morphological diversity; however, species delimitation has traditionally relied on light microscopy. In the present study, a septate gregarine parasitising Gonocephalum depressum from West Bengal, India, is investigated using DIC microscopy, SEM, and SSU rDNA sequencing. Trophozoites (446.98-596.15 μm) are elongated, bearing a distinctive stupa-shaped epimerite proper with a cylindrical diamerite and basal tumidus; epicytic folds (3-4 folds/μm) extend along the trophozoite except the epimerite proper. Gamonts (562.50-856.21 μm) are elongated with fold densities ranging from 3-4/μm to 4-4.5/μm. Associations are frontal. Gametocysts are roughly orbicular (178.51-219.73 μm), papillated, and dehisce by simple rupture. Oocysts (9.78-12.61 × 8.10-10.40 μm) are obovoid and smooth. Phylogenetic analysis of SSU rDNA places the species within Stylocephaloidea, where it forms a strongly supported clade (PP = 1; BP = 100) with congeners, confirming Stylocephalus epistupaformis n. sp. as a distinct species within Stylocephalus. The present study bridges a critical gap in septate gregarine research by providing the first integrative account combining light microscopy, SEM, and molecular phylogeny for a Stylocephalus species.